Recent developments in nanolithography techniques make it possible to produce submicron-scale spintronic devices based on magnetic nanostructures, such as spin valves, magnetic tunnel junctions and heterostructures based on magnetic semiconductors. Precise control of the magnetic configuration of such objects, such as the direction of magnetization or the structure of the magnetic domains, is one of the major technological challenges in the development of miniaturized spintronic devices.
Conventionally, the micromagnetic structure of a ferromagnetic element is composed of domains in which the magnetic moments are all aligned in the same direction, the domains being separated from one another by domain walls within which the magnetic moments gradually rotate. Thus, FIG. 1 shows a magnetic domain wall 3 separating two adjacent domains 1 and 2. The magnetic moments M are represented by arrows in bold. For the sake of clarity, the magnetic moments of the first domain are oriented in the opposite direction to the magnetic moments of the second domain in this figure. Of course, the magnetic moments of two domains may have different orientations. Within the domain wall, the orientation of the magnetic moments varies progressively, thus passing from the orientation of the first domain to that of the second domain. When a sufficiently high magnetic field is applied, the magnetic element no longer contains domain walls, and is called a one-domain element. To reverse the total magnetization, the direction of the applied magnetic field is reversed and the reversal of the magnetization then takes place by the nucleation and propagation of magnetic domain walls within the ferromagnetic element. In current devices, the external magnetic field is generated by a current flowing along lines close to the element. Controlled reversal of the magnetization of a ferromagnetic element used in spintronics corresponds for example to the writing of a magnetic bit.
The size of the domains and the number of magnetic domain walls present in the magnetic element depend on the dimensions of the ferromagnetic element. When the size of the ferromagnetic element decreases, it then becomes necessary to apply a more intense magnetic field for changing the magnetic domain walls. This phenomenon is particularly sensitive when the dimensions of the element are of the order of a few nanometers. Consequently, miniaturization within the nanoscale range of the magnetic bits obtained from ferromagnetic elements results in a large increase in the number of reversing magnetic fields necessary and consequently leads to an increasingly unacceptable consumption of energy for proper operation of the spintronic device. This phenomenon consequently limits the electronic storage capabilities associated with this technology.